1. Field
Example embodiments of the present invention may relate to a semiconductor memory device, and more particularly, to a resistive random access memory (RRAM) device.
2. Description of the Related Art
A resistive random access memory (RRAM) is a device having a variable resistive material, for example, a transition metal oxide. A variable resistive material has varying resistance depending on voltages applied thereto. When a voltage equal to or greater than a set voltage is applied to the variable resistive material, the resistance of the variable resistive material decreases, which is known as an ON state. Also, when a voltage equal to or greater than a reset voltage is applied to the variable resistive material, the resistance of the variable resistive material increases, which is known as an OFF state.
The conventional RRAM includes a storage node in which a lower electrode, a resistance variation layer, and an upper electrode are sequentially stacked. The RRAM usually includes a nickel oxide (NiOX) layer as the resistance variation layer. The conventional RRAM has two distinctively different resistance states. However, a disadvantage of the conventional RRAM is that the distribution of the voltage, at which the two resistance states change, is too broad. When the distribution of the voltage at which the resistance changes is too broad, it is difficult to obtain a resistance variation within a limited voltage range, because the resistance variation layer should have the same resistance state at the same voltage. However, the resistance variation layer may not have the same resistance state at the same voltage. Thus, the reliability of the conventional RRAM may be lowered.
In order to improve the RRAM, an RRAM having a storage node with a different composition has been introduced.
The storage node of this RRAM includes a lower electrode formed of copper (Cu) or silver (Ag), an upper electrode formed of platinum (Pt), and a solid electrolyte layer such as a CuS layer as a resistance variation layer formed between the lower electrode and the upper electrode. In the conventional RRAM, a current path may be formed in the solid electrolyte layer or the current path may be cut off depending on a voltage applied to the upper electrode and the lower electrode. The formation of the current path is due to Cu ions or Ag ions diffusing from the lower electrode into the solid electrolyte layer. The conventional RRAM is on or off depending on the presence of the current path. The voltage distribution of the RRAM is narrower than the previous RRAM.
However, in the RRAM, Cu ions or Ag ions are diffused into the solid electrolyte layer at such a high rate that the operation voltage of the conventional RRAM is too low.
FIG. 1 is a graph illustrating current-voltage characteristics of the RRAM. In FIG. 1, a section T1 illustrates where the current abruptly changes, that is, where the resistance abruptly changes.
Referring to FIG. 1, the resistance changes at about −0.3 V, indicating that the absolute value of the operation voltage of the RRAM is about 0.3 V. Therefore, since the operation voltage of the RRAM is very low, it is difficult to control the operation of the RRAM.
Also, in the case of a highly integrated device in which a size of a storage node is at a sub-micro level, a division between an upper electrode and a lower electrode vanishes due to excessive diffusion of the Cu ions or Ag ions from the lower electrode to the upper electrode, which may cause a malfunction of the highly integrated device.
In addition, Cu and Ag cannot be patterned using conventional etching methods such as a reactive ion etching (RIE) due to the characteristics of Cu and Ag, and thus it is very difficult to manufacture the RRAMs in which Cu or Ag is used to form the lower electrode.